1. Field of the Invention
The present invention relates to a glare-resistant touch panel, and more particularly relates to a technique for improving the operability of the touch panel in use environments that involve relatively high temperatures.
2. Description of the Related Art
A typical touch panel is constructed of an ultrathin glass plate (base sheet member) and a flexible transparent resin film (top sheet member) that are set facing each other and have resistive membranes respectively on their opposed surfaces, with spacers being placed in between the two sheet members in order to provide a certain spacing.
With an ever-widening range of applications of such touch panels, the demand for products that provide new additional values is growing in recent years. As an example, for combined use with a liquid crystal display (LCD) panel equipped in an outdoor-use device such as a car navigation system, a glare-resistant touch panel which is coated with a film of polarizer on its surface has been devised to keep the visibility of the display of the LCD panel from decreasing due to irregular reflections of incident light.
The polarizer used here is generally made of a resin film. Such a polarizing resin film has a thermal linear expansion coefficient eight to nine times that of glass which forms the base sheet member.
This large difference in thermal linear expansion coefficient between the top and base sheet members causes the following problems. As the temperature rises, the whole top sheet member including the polarizer bulges outward, thereby seriously deforming the touch panel. Besides, the operability of the touch panel decreases, as the space between the top and base sheet members widens and forces the user to apply a greater pressure to operate the touch panel. Given the recent trend toward larger touch panels with upsizing of LCD panels, the amount of outward bulge of the top sheet member is likely to increase, which will further degrade the touch panel""s operability.
This problem can be overcome by coating the upper surface of the polarizer with a resin film whose thermal linear expansion coefficient is lower than that of the polarizer, to keep the polarizer from bulging outward. An experiment on operability was conducted using a touch panel in which a PET (polyethylene terephthalate) film was adhered to the upper surface of the polarizer as a bulge-resistant film. As a result, the touch panel demonstrated high operability in a normal range of ambient temperatures. However, when left standing at a high ambient temperature over 60xc2x0 C. for a long time and then removed from that atmosphere and put in a normal temperature (around 25xc2x0 C.), the touch panel bulged upward and became seriously deformed, resulting in poor operability.
Inside a car parked in an outdoor parking lot under the scorching sun, the ambient temperature can easily reach as high as 80xc2x0 C. especially near a dashboard. If a car navigation system equipped with a touch panel is left in such a hostile condition, the top sheet member of the touch panel will bulge upward greatly, causing a significant decrease in operability.
In view of the above problems, the present invention aims to provide a touch panel whose appearance and operability are not impaired even after being left in high temperature environments, despite its use of a polarizer in a top sheet member to ensure visibility outdoors.
The above object can be achieved by a resistive-membrane touch panel including: a first sheet member having a first resistive membrane on a main surface thereof; a second sheet member that is flexible and has a second resistive membrane on a main surface thereof, the second sheet member being opposed to the first sheet member with a spacer interposed in between in such a manner that the second resistive membrane and the first resistive membrane face each other with a certain spacing therebetween; and a third sheet member that is laminated on an outer main surface of the second sheet member, wherein the second sheet member has a polarizing property, and the third sheet member has a thermal contraction rate no lower than a thermal contraction rate of the second sheet member.
Since the second sheet member has the polarizing property, the touch panel is guarded against glare. When such a touch panel is mounted on the front of a display panel such as an LCD panel, the visibility of the content displayed on the display panel is improved. Also, even when the second sheet member and the third sheet member contract in a high ambient temperature, after the ambient temperature returns to normal the third sheet member will not become slack against the second sheet member, because the thermal contraction rate of the third sheet member is equal to or larger than that of the second sheet member underneath it. Hence the outward bulge of the touch panel is effectively suppressed, and its appearance and operability is maintained regardless of temperature variations.
Here, the third sheet member may have a lower thermal linear expansion coefficient than the second sheet member. Even when the second sheet member is expanding to bulge outward as the ambient temperature rises, the third sheet member with the lower thermal linear expansion coefficient serves to suppress such an outward bulge of the second sheet member, so that the distance between the first and second sheet members will not widen substantially. As a result, the appearance and operability of the touch panel can be maintained regardless of temperature variations.
Here, the second sheet member and the third sheet member may be adhered with an adhesive that has a stress relieving property. The adhesive layer formed as such can absorb thermal stress growing between the second and third sheet members at elevated temperatures. Accordingly, no deviation from alignment will occur between the second and third sheet members, so that the operability of the touch panel can be maintained. Here, the viscoelasticity of the stress relieving adhesive is preferably in a range of 1.0xc3x97105dyn/cm2 to 1.2xc3x97106dyn/cm2. Also, the third sheet member may be coated with a hard coating material on its surface which is to be adhered to the second sheet member. In so doing, the adhesion of the third sheet member with the stress relieving adhesive is strengthened, which improves the durability of the touch panel.
Here, the third sheet member may have a lower hygroscopic linear expansion coefficient than the second sheet member. Even when the second sheet member is expanding to bulge outward due to moisture absorption, the third sheet member with the lower hygroscopic linear expansion coefficient serves to keep the second sheet member from bulging outward. Thus, the outward bulge of the touch panel due to moisture absorption can be effectively suppressed.